It is known to provide fluid sampling devices using optical near-field imaging as disclosed in U.S. Pat. No.5,572,320, which is incorporated herein by reference. Such a device is employed to determine quantity, size, characteristics, and types of particulate matter in fluids. Examples of fluids which are monitored in such a system are lubricating oils used in engines and rotating machinery; and fluids used in industrial quality control, food processing, medical analysis, and environment control. In its most common use, such a device monitors engine oil for metal particulates or flakes, wherein a size, number, and shape of particulates correspond to an engine condition and can alert one to particular problems with the engine. Predicting failure is critically important in aircraft engines to avoid accidents and loss of life.
The early stages of engine wear cause small particulate matter, of about 50 microns or less in size, to be generated. These particulates have characteristic shapes indicative of the type of wear produced by specific wear mechanisms. As the wear process progresses, the amount and size of particulates increase. Accordingly, sensing and identifying smaller particles allows early identification of faults, thus, allowing more time for corrective maintenance and preventing unexpected catastrophic failures.
Although current devices are sufficient in their stated purpose, several problems have materialized. For example, the flow pattern, which is analyzed through an optical flow cell carried by the sampling device, is not uniform. As such, non-uniform flow biases the particle distribution and may result in the inability to properly monitor the fluid and, in some cases, generate false positives. In other words, an inaccurate particle distribution may indicate that a problem exists with the engine when, in fact, there is not. Due to the small particle size and the positioning of the flow cell, the gap between the viewing plates and their positioning is of utmost importance. Current systems do not provide a repeatable frame of reference to provide adequate evaluation of the fluid as it flows through the optical flow cell. This is evident when the optical flow cells are damaged and must be replaced.
As will be appreciated by the discussion above, obtaining a uniform and consistent cell gap thickness, a positioning reference and a means of transitioning the fluid from a tube to the cell gap while maintaining a uniform flow is critical in obtaining consistent results over the life of the sampling device. Existing flow cells do not perform all of these functions, can be costly to manufacture, and can be prone to leakage. The molded device described here has the potential for being manufactured cheaply. Therefore, there is a need in the art for low cost replaceable optical flow cells which require minimal calibration and which maintain their characteristics for a long period of time.